Academics at Cambridge University have established a link with excess levels of the hormone cortisol. Researchers found that teenage boys who showed traditional depressive symptoms and had high levels of the hormone were 14 times more likely to be clinically depressed.

It means that for the first time, doctors could definitively identify the illness by a simple blood test. It also marks the first time that scientists have established that depression could be caused by a chemical imbalance.

Prof Joe Herbert, of Cambridge University, an author of the study, said the link could help identify young people at risk of "prolonged or even lifelong mental illness".

He added: "It also gives you some idea of causation, because these things aren't merely markers. They are having an impact on what is happening in the brain, no question about it."

Major, or clinical, depression affects one in six people at some point in their lives.

However, until now there have been no biomarkers for major depression; this is believed to be, in part, because both the causes and symptoms can be so varied.

Prof Ian Goodyer, of Cambridge University, and leader of the study said: "Through our research, we now have a very real way of identifying those teenage boys most likely to develop clinical depression.

"This will help us strategically target preventions and interventions at these individuals and hopefully help reduce their risk of serious episodes of depression and their consequences in adult life."

The researchers measured levels of cortisol in saliva from two groups. The first consisted of 660 teenagers, who provided four early-morning samples on school days within one week and then again 12 months later. The researchers were able to show that within this group cortisol levels were consistent over one year in the population at large in both boys and girls.

A second group, of 1,198 teenagers, provided early-morning samples over three school days. Using self-reports about symptoms of depression collected over the 12 months and combining these with the cortisol findings, Prof Goodyer and colleagues were able to divide the teenagers in the first group into four subgroups.

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They ranged from those with normal levels of morning cortisol and low symptoms of depression over time – Group 1 – through to teenagers with elevated levels of morning cortisol and high symptoms of depression over time – Group 4. This latter group made up one in six of all subjects, or 17 per cent.

Because the two groups gave identical results, Prof Goodyer and his colleagues were able to combine them and study the whole sample of 1,858 teenagers for the probability of developing clinical major depression and other psychiatric disorders 12 to 36 months later.

Analysis showed that boys in Group 4 were 14 times more likely to suffer from major depression than those in Group 1 and two to four times more likely to develop the condition than either of the other two groups.